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54
pH
% Protonated (-NH
3
+
)
% Un-protonated (-NH
2
)
pK
a
+ 2
1
99
pK
a
+ 1
10
90
pK
a
(near 6.5 for
chitosans)
50
50
pK
a
- 1
90
10
pK
a
- 2
99
1
If the chitosan is supplied in the ‘free amine’ form (-NH
2
) then acid must be
added to convert it to a soluble form. If pH is increased again the chitosan will
re-precipitate.
So-called ‘water-soluble’ chitosans are in fact chitosans that have been dried
directly from the soluble (i.e. acidic) state. In these cases the acid ‘follows’ the
chitosan. Thus, if HCl (hydrochloric acid) was used to dissolve it, the dried
form is chitosan (hydro)chloride: -NH
3
Cl. When water is added, most of the
amino groups remain in the –NH
3
+
form, which is the soluble form
19
. Some
(not all, remember –NH
3
+
is a weak acid) of the protons dissociate from the
chitosan, and pH remains low enough for the chitosan to remain in solution.
19 Thermodynamically, the driving force is mainly the increase in entropy (
Δ
S) when the counter-ion dissociates.